This application is a 371 application of PCT/JP99/06974 filed Dec. 13, 1999.
The present invention relates to a structure comprising a thermoplastic resin. More specifically, it relates to a thermoplastic resin structure having a three-layer structure of flexible skin layer/flexible foamed material layer/hard substrate layer, which is excellent in cushioning and strength, and makes it possible to effectively reuse, i.e., recycle the used articles and to reduce costs.
Known three-layer structures of flexible skin layer/flexible foamed material layer/hard substrate layer are shown as follows.
Conventional technique 1: a structure which comprises a slash molded product of polyvinyl chloride (hereinafter abbreviated as PVC) or a polyolefin thermoplastic elastomer (hereinafter abbreviated as TPO) as a flexible skin layer (hereinafter referred to as a skin layer), a polyurethane-foamed product as a flexible foamed material layer (hereinafter referred to as a foamed layer), and a polypropylene composite material or an injection-molded product of an ABS resin as a hard substrate layer (hereinafter referred to as a substrate layer).
Conventional technique 2: a structure which comprises a polypropylene composite material or an injection-molded product of an ABS resin as a substrate layer, and a sheet of a two-layer structure obtained by laminating a cross-linked foamed sheet of polypropylene or high density polyethylene as a foamed layer on a skin layer of PVC or TPO.
These structures have been used for automobile interior materials and have contributed greatly to the industrial fields as having excellent appearance, cushioning it, and rigidity.
In the conventional technique 1, however, foamed polyurethane, i.e., a thermosetting resin is used for the foamed layer, so that the skin layer, the foamed layer and the substrate layer should be separated when the structure is reused, i.e., recycled. Further, it is difficult to reuse foamed polyurethane, and therefore, the structure is not suitable for recycling which has been required worldwide in recent years.
The conventional technique 1 also has a disadvantage of high production cost, caused by quite different facilities required in the respective steps of a slash molding step of a skin layer, an injection or injection press molding step of a substrate layer and a urethane discharge-foaming step of a foamed layer.
In the conventional technique 2, a cross-linked foamed material is used for the foamed layer. When the three layers are recycled at the same time, the cross-linked foamed sheet is not homogeneously molten with materials constituting the skin layer and the substrate layer. The performance of the structure is therefore reduced when used as a recycled material. That is, the structure has a difficulty in recycling.
The conventional technique 2 also has a disadvantage of high production cost, caused by a long series of molding steps comprising a foamed sheet-molding step of a foamed layer, a step of laminating a skin layer on the foamed sheet, a step of pre-molding the resulting composite sheet by a vacuum molding method or a compress molding method, and a step of combining the resulting pre-molded product with a substrate layer to mold in a die by an injection or injection press molding method, and by the poor yield in pre-molding of the expensive composite sheet.
Further, both conventional techniques 1 and 2 described above use an adhesive or a pressure-sensitive adhesive so that at least a part of the respective layers adhere to each other, which may also cause difficulty in recycling of the structures and high costs.
The problems of difficulty in recycling and a high production cost in these conventional techniques are caused by that the layers, respectively having a good appearance, cushioning and rigidity, are produced from quite different materials or by quite different processing methods and combined. Such a structure may be superior in quality, but the above problems cannot be solved.
An object of the present invention is to provide a thermoplastic resin structure capable of being surely recycled and reducing costs while maintaining high performance in appearance, cushioning and rigidity, which has not been achieved by conventional techniques.
The present inventors have made intensive researches in order to obtain a thermoplastic resin structure capable of being surely recycled and reducing costs while maintaining high performance in appearance, cushioning and rigidity, which has not been achieved by conventional techniques. As a result, the inventors have found that a structure having a three-layer structure of a flexible skin layer, a flexible foamed material layer and a hard substrate layer, the respective layers comprising related thermoplastic resins capable of melt adhesion with each other, the flexible foamed material layer comprising an injection foam molded product, can be surely recycled and reduce costs, wherein the skin layer and the substrate layer can be combined at the same time of molding the foamed material layer, and this three-layer structure can easily be molten and reused only by pulverizing without separating the respective layers, whereby the present invention has been completed.
The constitution of the present invention is as follows.
(1) A thermoplastic resin structure having a three-layer structure of flexible skin layer/flexible foamed material layer/hard substrate layer, wherein the respective layers comprise thermoplastic resins capable of melt adhesion with each other and the flexible foamed material layer is formed by an injection foam molding method.
(2) The thermoplastic resin structure as described in the item (1), wherein the flexible skin layer and the hard substrate layer in the three-layer structure comprising the flexible skin layer, the flexible foamed material layer and the hard substrate layer are each thermally welded or thermally fused with the thermoplastic resin constituting the flexible foamed material layer by heat and pressure in injection foam molding of the flexible foamed material layer.
(3) The thermoplastic resin structure as described in the item (1) or (2), wherein the flexible skin layer, the flexible foamed material layer and the hard substrate layer in the three-layer structure each comprise polyolefin thermoplastic resins.
(4) The thermoplastic resin structure as described in any one of the items (1) to (3), wherein the flexible foamed material layer has an average expansion coefficient of 1.2 to 10.